1. Field of the Invention
The present invention relates to a scanning optical device and, more particularly, to a scanning optical device suitably used for image forming apparatuses such as a laser beam printer and digital copying machine, in which a light beam emitted from a light source means is guided onto a scanned surface as a recording medium surface through a light deflector such as a rotating polygon mirror, and the light beam is scanned on the scanned surface, thereby recording characters, information, and the like.
2. Related Background Art
In a conventional scanning optical device used for a laser beam printer, a digital copying machine, and the like, a light beam emitted from a light source means is deflected by a deflection means, and the deflected light beam forms a spot on a photosensitive drum surface as a scanned surface via a scanning optical means, thereby scanning the light beam on the scanned surface.
FIG. 1 is a schematic view of the main part of a conventional scanning optical device. Referring to FIG. 1, a light beam emitted from a light source means 11 constituted by a semiconductor laser and the like is converted into a substantially collimated light beam by a collimator lens 12. This substantially collimated light beam is shaped into an optimal beam shape by an aperture stop 13 and strikes a cylindrical lens 14. The cylindrical lens 14 has a power in the sub-scanning direction and forms a light beam image elongated in the main scanning direction near a deflection surface 15a of the light deflector 15 constituted by a rotating polygon mirror and the like. In this case, the main scanning direction is a direction perpendicular to the deflection scanning direction, and the sub-scanning direction is a direction perpendicular to the deflection scanning direction. This applies to the following description. The light beam is reflected/deflected by the light deflector 15 at an equal angular velocity to form a spot on a photosensitive drum surface (recording medium surface) 18 as a scanned surface via an f.theta. lens 16 which is a single element lens serving as a scanning optical means having an f.theta. characteristic. This light beam is scanned on the photosensitive drum surface 18 at an equal velocity.
In a scanning optical device of this type, a rotating polygon mirror having many deflection surfaces (reflection surfaces) is generally used as a deflection means, and an optical axis is set at a given angle before and after deflection in the overall scanning optical system. That is, a light beam emitted from a light source means is obliquely incident on the deflection means within a scanning plane. For this reason, deflection surfaces asymmetrically enter and exit within one scanning operation with reference to on-axis scanning. The influences of this asymmetry are noticeable in a sub-scanning cross-section in which a light beam is temporarily imaged near the deflection surface to set the deflection surface and the scanned surface conjugate with each other. This makes it difficult to correct curvature of field.
To realize a scanning optical device with a higher resolution (smaller spot), the influences of asymmetry on curvature of field and f.theta. characteristics in the main scanning direction must be considered. As a means for solving these problems, for example, the scanning optical devices disclosed in Japanese Laid-Open Patent Application Nos. 8-122635 and 8-248308 use a means for asymmetrically changing this radius of curvature in the sub-scanning direction with respect to the optical axis. This allows proper correction of curvature of field in the sub-scanning direction throughout the entire scanning width. A lens having such a shape is generally manufactured by plastic molding in consideration of technical and economical advantages.
As is known, however, plastic materials are susceptible to environmental variations. With a change in ambient temperature, in particular, the refractive index of a plastic material changes. As a consequence, the focal point of the plastic lens varies, and for example, the spot size on a scanned surface increases. This makes it difficult to obtain a high-quality printed image.